Conventionally, when a large workpiece is turned, a large-sized lathe is generally used. However, if the workpiece is not in good balance, the rotation rate of the workpiece is limited. This requires a great reduction in the cutting speed conditions such as the feed rate. To deal with this drawback, a counterweight may be attached to the workpiece so as to improve the balance. However, this countermeasure increases the energy consumption due to the addition of the counterweight when the workpiece is rotated, and limits the weight of the workpiece.
Hereinafter, a prior art technique for turning a large workpiece will be described.
A workpiece W shown in FIG. 12 is a large workpiece having a weight of 1160 kg. The machining diameter of a hole Wa to be formed through boring is 730 mm.
When the workpiece W is machined by being revolved about a center O1, which is displaced from the center of gravity O of the workpiece W, the maximum diameter of the swing of the workpiece W is 2330 mm as shown in FIG. 12 (the radius from the center O1 being 1165 mm). In the example of this workpiece W, since the center of gravity O is at the position of 237 mm away from the revolution center, the rotation of the workpiece W is imbalanced.
To perform such machining, a significantly large vertical lathe is necessary. In other words, the turning of the workpiece W needs to be performed by a different large vertical lathe, which significantly extends the overall machining time.
Accordingly, to avoid such a turning process, circularity machining is performed as shown in FIG. 13. The circularity machining refers to a process in which a workpiece W fixed onto a table is moved along a hole Wa while an end mill is rotated. The circularity machining allows machining of other parts of the workpiece W to be performed with the same machine tool. However, the machining accuracy of the circularity machining is inferior to that of a turning process.
As described above, conventionally, a large workpiece that is difficult to rotate is machined in the following manner. That is, such a workpiece is subjected to the circularity machining by an end mill, which sacrifices machining accuracy and machined surface. The workpiece is also subjected to a turning process using a horizontal boring machine, or a boring process performed by rotating the workpiece with a large vertical lathe in the knowledge of a resultant imbalance. In other words, machining processes are switched, which requires setup of workpiece and tools each time.
Patent Document 1 discloses boring by a horizontal boring machine.